(1) Field of the Invention
The present invention performs status monitoring of a feedforward cancellation power amplifier by applying levels of radio frequency (RF) monitor signals within the feedforward cancellation power amplifier. The on-line monitoring of the monitor signals can be performed without significant radiation of monitor signal power from a load antenna because the inherent properties of the feedforward cancellation power amplifier are employed and, as a result, the monitor signals are cancelled to a very low level in the output. The monitor signals are used to measure the status of all individual components of the amplifier dealing with RF signals, such as power amplifiers, distortion cancellation amplifiers, and output circuit components.
(2) Description of the Related Art
Prior art systems for status monitoring use operational communication signals for RF monitoring. The signals have the disadvantage of a large dynamic range necessitating complex RF detection equipment. They are also not suitable for monitoring distortion amplifiers.
Ultra-linear high power amplifiers employ feedforward cancellation to achieve high linearity. Feedforward cancellation employs distortion amplifiers to generate signals that cancel distortion and noise in the output of the ultralinear amplifier. Two cancellation loops are usually employed for a single stage. For example, as set forth in U.S. application Ser. No. 07/291,605 entitled "A Gain Variation Compensating Circuit for a Feedforward Linear Amplifier", by Terence E. Olver, (incorporated by reference herein), two cancellation loops are used for a single stage. Loop 1 includes a main amplifier which is the principle source of power amplification and serves to separate distortion and noise power generated by the main amplifier from fundamental input signals. This is accomplished by subtracting a sample of the output power taken at the output sampling coupler and attenuator, from a sample of an input signal. The input signal sample is taken by way of the input sam coupler, matching circuit and delay line. Actual subtraction is performed in the fundamental cancelling coupler. Careful phase and amplitude matching of the two samples fed into the cancelling coupler must be maintained across a frequency band of approximately 2 to 30 MHz for high frequency power amplifiers. The quality of matching determines the residue of the fundamental signal left after subtraction in the output of the fundamental cancelling coupler. This output, containing phase inverted distortion and noise signals, plus the fundamental residue, forms an input to the distortion amplifier where it is amplified and then injected into the distortion cancelling coupler.
Loop 2 includes two signal paths which are carefully matched in phase and gain. The distortion cancelling signal path is from the output sampling coupler by way of an attenuator, fundamental cancelling coupler, distortion amplifier and a distortion cancelling coupler. The high power signal path is from the output sampling coupler by way of the delay line and the distortion cancelling coupler. Matching is performed so that across the frequency band the distortion and noise signals injected into the output from the distortion cancelling path of loop 2 are equal in amplitude, but are 180.degree. out of phase with those signals coupled from the main amplifier directly by way of the delay line. As a result, distortion and noise are cancelled in the output. Also directly coupled to the output are the high power fundamental signals providing the power output, which are not cancelled.
Phase matching is performed by carefully adjusting the time delay in both paths so that the phases are equal. The delay line is the main component used for phase matching. Gain matching is performed by adjusting the gain of the distortion amplifier for optimum broad band conditions so that the amplitude of the signals is equalized. The gain of the distortion amplifier and the stability of its operation are therefore critical to the degree of distortion cancellation in the output and thus, to the linearity of the distortion amplifier. The gain monitoring of the power amplifiers that form the main amplifier is performed on-line by monitoring and comparing the gain between the input and output signals. This is relatively simple to perform since the amplified signals are usually large (many watts) for high power amplifiers and small samples sufficiently large to operate detectors are readily obtained.
Communication traffic signals are used for on-line monitoring but these signals have a large dynamic range, e.g., up to 50 dB. This is not acceptable for a distortion amplifier. In a distortion amplifier, signals consist of cancelled fundamental residue plus distortion and noise. At the input to the distortion amplifier the largest signals are approximately 1 milliwatt. During normal traffic conditions, the signals will be much smaller and may extend downward over a dynamic range of approximately 50 dB. It is difficult to sample and detect such signals and thus determine the gain of the distortion amplifier.
The present invention uses special in-band signals strictly for monitoring purposes. In the past, there has been objection to using in-band signals because injection of the signals into the amplifier can result in coupling to the output with the signals being radiated from any antenna connected to the amplifier output. Use of a high power switch to disconnect the output from the antenna is not possible for the reason that a high power very linear changeover switch is not available. In addition, such a switch would be costly and less reliable than not using a switch.